The effects of COVID-19 remained significant on university students' social and academic well-being. The pandemic raised various mental and psychological challenges for the students, and had the potential to deteriorate their academic engagement and performance in online learning environments. The emotion regulation theory hypothesizes that various cognitive skills including cognitive reappraisal may assist people in coping with such types of social and psychological challenges. This study aimed to investigate the influence of cognitive reappraisal "as a mediator" on the relationships between COVID-19 stressors and university students' engagement in an online learning environment. Three hundred and one students selected from eight public sector universities in Pakistan participated in a survey. Three instruments, including the COVID-19 Related Stressors, Cognitive Reappraisal Questionnaire, and Student Engagement Dimensions, were used to collect data. Mediation analysis confirmed the application of emotion regulation theory in the context of relationships between COVID-19 stressors and students' engagement in online learning environments. The study has significant implications for promoting students' engagement in online setups during stressful situations.